Method of filling a casing with heat insulating fibers

ABSTRACT

Method of filling a casing with heat insulating fibers in which a fibrous heat insulating mass of fixed size is inserted into a space to be filled by vacuum-packing the fibrous mass in a vacuum-resistant bag, which may then be wrapped about an inner cylinder, introducing the bag into an outer cylinder and, after breaking the vacuum seal, allowing the fibrous mass to swell and fill any space within the outer cylinder not occupied by said inner cylinder, if any.

BACKGROUND OF THE INVENTION

Automobile exhaust gas purifers such as manifold reactors or catalyticconverters should comprise a heat insulator which can withstand hightemperature, because the inside of such a device has to be kept warm sothat its exhaust gas purifying ability may be improved and the heatreleased from such a device has to be prevented from affecting adjacentparts of the automobile.

The so-called ceramic fibers, which are fibrous heat insulators for hightemperature use constitute one of the materials available for thispurpose. Fibers of alumina-silica can withstand a maximum workingtemperature of 1200°-1400° C.; one of silica can withstand 1000° C.; oneof potassium titanate can withstand about 1000° C.; and one of zirconiacan withstand 1800° C. Slag wool is also available, but the workingtemperature it can withstand is low, i.e., about 600° C.

These fibrous materials have a heat insulating ability two to threetimes as high as that of heat resistant, heat insulating brick; a bulkspecific gravity of 0.05-0.25 g/cm³, which is about 1/8 of that of theheat insulating brick; and are flexible and vibration-resistant, so asto be quite free from the possibility of being broken by heat shock.Being less resistant to wind velocity, however, they usually need aheat-resistant metal plate applied on the heating surface and aresandwiched between the metal plate and the outer shell, when they areused in a manifold reactor. When they are used in a catalytic converter,they fill the space between the catalyst carrier and the outer shell. Inany case, the fibrous heat insulator has to fill a very narrow space. Todo this efficiently without sacrificing performance, various methodshave been worked out. To give some examples, there are:

1. The method of inserting a bulky heat insulator through the end of thespace between the heat insulating inner cylinder and outer cylinder;

2. The method of introducing into the outer cylinder an inner cylinderwrapped with a felt-like heat insulator, or sheathing the inner cylinderwith an outer cylinder split into two parts; and

3. The method of inserting a stainless steel, foil-packed fibrous heatinsulator. There are, however, many drawbacks in these methods. Forexample:

1. The efficiency is poor;

2. The fill density becomes uneven; and

3. It is expensive.

SUMMARY OF THE INVENTION

The present invention provides a method of filling a narrow heatinsulating space in an exhaust gas purifier such as a manifold reactor.According to the present invention, the reactor can be filled with afibrous heat insulator with extremely high efficiency and uniformity,thereby substantially increasing the work efficiency. Moreover, sincethe space can be filled to a high density with the fibrous heatinsulator, its heat insulating capacity can be improved and accordinglythe purifying performance of the exhaust gas purifier can be increased,while the amount of heat released can be decreased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the assembly comprising the engine, the manifold reactor,the exhaust pipe and the muffler.

FIG. 2 is a perspective view of the manifold reactor fitted to theengine, with part of the reactor wall shown broken away.

FIG. 3 is a longitudinal sectional view taken through the manifoldreactor.

FIG. 4 is a sectional view taken along the line II--II' of FIG. 3.

FIG. 5 is an oblique view of a heat insulator with specified portionsstamped out.

FIG. 6 is an oblique view of the heat insulator of FIG. 5 asvacuum-packed.

FIG. 7 is a partial sectional view taken through a vacuum-packed heatinsulating inner cylinder attached to a heat insulator.

FIG. 8 is a sectional view of a heat insulating cylinder accessory.

FIGS. 9-14 are diagrams showing the thickness of a heat insulator whenit is vacuum-packed and when the vacuum seal is broken.

FIG. 15 is a diagram showing the thermal conductivities of ceramic fiberblankets with various densities.

FIG. 16 is a partially fragmented oblique view of a heat insulatedexhaust pipe.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, which relies on the compressibilityof a fibrous heat insulating material, the fibrous heat insulatingmaterial is placed in a heat-resistant film bag; the bag isdepressurized to compress and reduce the volume of the fibrous heatinsulator; the heat insulating material which has been thus compressedand reduced in volume is inserted into the space to be insulated fromheat; and then the vacuum seal of said bag is broken to expose itscontents to atmospheric pressure, so that said insulating materialexpands to fill said space.

According to the present invention, the same purpose may also beattained by placing fibrous heat insulating material, which has been cutto the size of a space to be filled, into a vacuum-resistant film bag;vacuum-packing the resulting assembly; inserting it into the space to befilled; and then breaking the seal of the package. In the case of amanifold reactor or a catalytic converter having an intricateconfiguration of inner and outer cylinders, unlike the case of anordinary heat insulated pipe in which a heat insulator is introducedbetween outer and inner cylinders having smooth surfaces, it would beadvisable to apply a laminated layer of the fibrous heat insulatingmaterial on the surface of the inner cylinder, insert the whole innercylinder into a film bag which is then hermetically sealed forvacuum-packing; and then break the seal of the vacuum-package afterhaving introduced it into the outer cylinder.

As understood from the above, the hermetically sealed bag to be used inthe present invention should desirably be flexible and for this purposethe flexible plastic film should be one that does not break whenvacuum-packed. For instance, nylon, polyethylene, polypropylene films,etc. may be usd singly or as a laminated sheet. Preferably, the filmshould vanish when burned.

In addition to the above-mentioned ceramic fiber as fibrous heatinsulator, anything fibrous with elasticity may be used for the presentinvention.

Tables 1-3 give the characteristic values of a few examples of thefibrous heat insulator. FIGS. 9-14 show the thicknesses of thesematerials when they are vacuum-packed under various pressures and whentheir hermetic seal is broken. FIG. 15 shows the thermal conductivitiesof ceramic fiber blankets with various densities.

                  TABLE 1                                                         ______________________________________                                        Characteristic values of                                                      ceramic fiber blankets                                                        Item                 Characteristic values                                    ______________________________________                                        Fiber diameter           2.8μ average                                      Fiber length             100 mm average                                       True specific gravity    2.56                                                 Melting point            1760° C.                                      Chemical        Al.sub.2 O.sub.3                                                                       50.1%                                                Composition     SiO.sub.2                                                                              49.1%                                                                Fe.sub.2 O.sub.3                                                                       0.2%                                                                 TiO.sub.2                                                                              0.2%                                                                 CaO      0.1%                                                                 MgO      Trace                                                                Na.sub.2 O                                                                             0.3%                                                 ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Characteristic                                                                value of rock wool*                                                           Items                Characteristic values                                    ______________________________________                                        Fiber diameter           4-6μ                                              Density                  0.11-0.15 g/cm.sup.2                                 Particle-content         less than 3%                                         Working temperature      -200-800° C.                                  range                                                                         Chemical        SiO.sub.2                                                                              35-45%                                               Composition     Al.sub.2 O.sub.3                                                                       10-15%                                                               CaO      30-40%                                                               MgO      5-7%                                                 ______________________________________                                         *S-fiber produced by ShinNihon Seitebu Kagaku                            

                  TABLE 3                                                         ______________________________________                                        Characteristic                                                                values of silica fiber*                                                       Items               Characteristic values                                     ______________________________________                                        Fiber diameter          1.3μ average                                       Fiber length            20 mm average                                         True specific           2.5%                                                  gravity                                                                       Melting point           1713° C.                                       Chemical      SiO.sub.2 98%                                                   Composition   Na.sub.2 O                                                                              0.3%                                                                Others    1.7%                                                  ______________________________________                                         *product of Nibor Glass Fiber Co.                                        

FIG. 9 illustrates the thickness of a rock wool mass, initiallymeasuring 20 mm thick, 100 mm wide and 100 mm long, after vacuum-packingunder various vacuum pressures. As seen therefrom, the thickness of therock wool is reduced to about 1/3 of its initial value. The term "vacuumpressure" as used here means the difference between atmospheric pressureand the pressure attained after the maximum depressurization. As shownin FIG. 10, when the hermetic seal is broken, the thickness of the massis restored at most to about twice the thickness when vacuum-packed. Thevacuum-resistant film used was a 50μ thick polyethylene film laminatedto a 15μ thick nylon film. The same film was used in all other cases.

FIG. 11 illustrates the thicknesses of ceramic fiber blankets (as listedin Table 1) with various densities measuring 12.5 mm thick, 100 mm wideand 100 mm long, after vacuum-packing under various vacuum pressures.

FIG. 12 illustrates the thicknesses of those blankets in FIG. 11 afterthe hermetic seal is broken. As seen from FIG. 11, vacuum-packingreduces the thickness of the ceramic fiber blanket to as little as 1/4of the initial value; when the hermetic seal is broken, a substantialincrease from the vacuum-packed thickness takes place as indicated inFIG. 12.

FIGS. 13 and 14 respectively show the vacuum-packed thickness and thevacuum-broken thickness of the ceramic fiber blankets in FIGS. 11 and 12when they are initially made 25 mm thick.

As described above, the fibrous heat insulator can be compressed to afraction of its original thickness by vacuum-packing. When the vacuumpressure is 1 kg/cm², the compressive pressure rises to a maximum, i.e.,1 kg/cm².

A vacuum-packed fibrous heat insulator, when the vacuum has been broken,swells from several tens to one hundred percent in the direction of itsthickness. Therefore, when the vacuum of a vacuum-packed heat insulatorthat can swell to twice its vacuum-packed thickness is within a space1.5 times the thickness of the insulator, the insulator will swell tofill the space, and still have an extra capacity to expand. Thus withhigh density retained, the heat insulator will have a low thermalconductivity, an excellent insulating performance and excellentresistance to vibration, as illustrated in FIG. 15. There is anotheradvantage, in that even when the dimensions of the inner or the outercylinder change due to the temperature variations, the insulator cancorrespondingly swell, showing no great change in heat insulatingperformance.

Several specific embodiments of the present invention will now bedescribed.

EXAMPLE 1

In FIGS. 1-4 showing the engine, manifold reactor, exhaust pipe andmuffler reference numeral 1 indicates the engine; 2 indicates themanifold reactor in which CO and HC among the harmful emissions from theengine 1 are burned and transformed into harmless CO₂ and water, and 3indicates the exhaust pipe which carries the exhaust gas out of theexhaust port of the manifold reactor 2 to the muffler 4.

In the engine 1, 1e is the combustion chamber of the engine, in whichthe gasoline and the air react with each other in an explosion whichgenerates the exhaust gas. When the exhaust valve 1a opens, the exhaustgas passes out through the exhaust port 1f. Reference number 1gindicates the valve seat, 1c the water-cooled jacket for cooling thecylinder head 1b. 1d indicates an air inlet pipe through which an airpump introduces air to the exhaust port 1f to promote the re-combustionof the exhaust gas expelled through the exhaust port 1f, and 1hindicates a gasket. A mixture of the air introduced through the air pipe1d and the exhaust gas passes into the cylindrical exhaust gas inlet 5of the manifold reactor 2 and then enters the cylindrical combustionchamber 6. The exhaust gas which has been burned again in the combustionchamber 6 passes through the cylindrical gas outlet 19 into the exhaustpipe 3, and, with the noise muffled by the muffler 4, passes out of thetail pipe 4'.

In the above arrangement the gas temperature in the re-combustionchamber 6 reaches 900°-1000° C. In order to shield the surrounding partsfrom this heat, the space between the inner cylinder 7 and the outercylinder 8 is filled with fibrous heat insulators 9, 9'. Referencenumeral 10 indicates a cylindrical heat insulating duct for preventingthe material of the heat insulators 9, 9' from dispersing into theexhaust gas. This duct is welded to the inner cylinder 7 and the outercylinder 8 at the exhaust gas inlet and outlet.

The method of filling the heat insulators 9, 9' in this embodiment willnow be described.

A ceramic fiber blanket having the properties indicated in Table 1 (aproduct of Isolite Industry K.K., trade name "Kao-wool blanket," havinga density of 0.128 g/cm², and a thickness of 12.5 mm) is cut into apiece of such size that it can be wrapped around the heat insulatinginner cylinder 7. After cutting a hole therein for the heat insulatingduct 9b of the exhaust gas inlet and for the heat insulating duct 9c ofthe exhaust gas outlet, a blanket 9a, as illustrated in FIG. 5 isobtained. This blanket 9a is wrapped around the inner cylinder 7 whichhas the side cover 7' of the inner cylinder and the heat insulating duct10 welded thereto, and the end faces 9e, 9e' of the blanket are joinedtogether and attached by means of a tape or the like. In addition, ablanket heat insulator 9' of the disk type is prepared with a holeprovided therein for receiving the bolt 16 to support the inner cylinder7 and this is pressed and fitted against the end cover 7' of the innercylinder.

The inner cylinder 7, thus firmly wrapped in a blanket, is placed in avacuum-resistant bag (such as the one mentioned above) and vacuum-packedunder a vacuum pressure of 1 kg/cm². For this purpose, a vacuum-packerModel A-450-L produced by Furukawa Seisakusho is employed; and thismachine is also used in the following examples.

FIG. 7 illustrates the reactor in a vacuum-packed state. In FIG. 7,reference numeral 20 is a vacuum-resistant bag, 21 is the hermetic seal,and 21' is the sealed bottom of the bag. As a result of such avacuum-sealing, the thickness of the heat insulating layer can bereduced to 4.9 mm, including the thickness of said vacuum-resistant bag.

The vacuum-packed product illustrated in FIG. 7 is introduced into theouter cylinder 8 and heated at 500° C. for 30 minutes to burn away thevacuum-resistant bag. Thus released from vacuum, the blanket swells anduniformly fills the 8 mm gap between the inner and outer cylinders 7, 8.Then the heat insulating duct 10 and the heat insulating outer cylinder8 are welded together, and the end cover 8' is welded to the outercylinder 8, thereby completing the heat insulating cylinder accessory(FIG. 8). The blanket 9' may be inserted in a conventional way beforethe end cover 8' is welded to the outer cylinder 8.

The heat insulating cylinder accessory constructed in this manner isinserted into the outer cylinder 11; the end cover 15 and gasket 18 areattached; the support bolt 16 for the inner cylinder 7 is inserted andthe end cover 15 is bolted by the bolt 17 to the outer cylinder 11.

Thereafter the manifold reactor is completed by providing the ducts 13,14 (see FIG. 3). This reactor is then bolted to the engine by means ofthe bolt 12.

Among the components of the manifold reactor, the inner cylinder 7, theouter cylinder 8, the ducts 13, 14 and the bolt 16 are made of stainlesssteel. JIS-SUS-310S; the bolt 12 is made of stainless steel JIS-SUS-304:and the outer shell 11 and the end cover 15 are made of cast iron(FCG-23).

EXAMPLE 2

The same ceramic fiber blanket as in Example 1 is used and by subjectingit to the same treatment as in Example 1, a blanket 9a (FIG. 5) to bewrapped around the inner cylinder 7 is prepared.

This blanket 9a is placed in a vacuum-resistant bag 20 (the same asabove) of polyethylene laminated to nylon, and is vacuum-packed. Thevacuum-packed product has its parts corresponding to the heat insulatingducts 9b, 9c for inlet and outlet of the exhaust gas in the blanket 9aheat-sealed; and with openings for receiving the heat insulating ductsstamped out, a vacuum-package as illustrated in FIG. 6 is obtained.

This vacuum-package is wrapped around the inner cylinder 7, fastenedwith a tape or the like and inserted into the outer cylinder 8, afterwhich it is treated as in Example 1, thereby making a heat insulatingcylinder accessory and completing a manifold reactor.

In this embodiment the heat insulator 9' to fill the end of the heatinsulating cylinder may be vacuum-packed before insertion just as inExample 1, or it may be inserted in a conventional way.

In the present example, in which only the blanket is vacuum-packed, itsoriginal thickness of 12.5 mm can be reduced to 4.5 mm, including thethickness of the bag, which is thinner than in Example 1.

A heat insulating cylinder accessory prepared as in Examples 1 and 2 wascompared in a vibration test with a heat insulating cylinder accessoryprepared by inserting a 7 mm thick ceramic fiber blanket of the samequality as in Example 1 by a conventional method. The test conditionswere as described below, and after the test, each accessory was cut openfor investigation. The results shown that the products of Examples 1 and2 were uniformly filled, but the conventional product had its blanketloosened, bulky and bent toward the bottom of the accessory.

    ______________________________________                                        Test conditions                                                               ______________________________________                                        Frequency:        90 Hz                                                       Vibrational acceleration:                                                                       45 G                                                        Amplitude:        about 2 runs                                                Test time:        5 hours                                                     Test apparatus    electromagnetic vibrators                                                     tester                                                      Vibrational directions:                                                                         normal to the diameter                                                        of heat insulating                                                            cylinder and up and down                                    ______________________________________                                    

EXAMPLE 3

In this example, a method of filling a heat insulated exhaust pipe isdescribed, a partially cut away oblique view thereof being shown in FIG.16, with a heat insulator. In FIG. 16, reference numeral 22 indicates anouter cylinder made of JIS-STKM-11 steel. 23 an inner cylinder made ofJIS-SUS-304 steel, and 25 a flange made of JIS-SUS-304 steel, while 26indicates the bolt hole.

First, a rock wool pad, 0.14 g/cm² in density and 20 mm in thickness (aproduct of Shin-Nippon Seitetsu Kagaku, see Table 2) is wrapped aroundthe inner cylinder 23, which has a flange 25 welded thereto at 23a; thebutt joint is firmly taped; and the resulting assembly is inserted intoa heat resistant film bag (the same type as in Example 1). It is thenvacuum-packed to a vacuum pressure of 1 kg/cm². Vacuum-packing reducesthe thickness of the wool pad to 6.4 mm.

Next the resulting vacuum-package is introduced into the outer cylinder22, and heated at 500° C. for 20 minutes to burn away thevacuum-resistant film bag. Thus released from the vacuum, the heatinsulator 24 fills the space between the inner and outer cylinders.

The outer diameter of the inner cylinder is 40 mm, the inner diameter ofthe outer cylinder is 56 mm, and the thickness of the heat insulatingspace between the two cylinders is 8 mm.

EXAMPLE 4

A 25 mm thick rock wool pad of the same quality as in Example 3 is usedas the heat insulator. A layer of this wool is cut into a piece ofspecified size, which is inserted into a vacuum-resistant bag (of thesame type as above) and vacuum-packed, to a vacuum pressure 1 kg/cm².Thus vacuum-packed, the thickness, including that of the bag, can bereduced to 6.5 mm.

Next the resulting vacuum-package is wrapped around the inner cylinder23 having the flange 25 welded thereto, the butt joint is firmly taped,the inner cylinder 23 thus treated is introduced into the outer cylinderand heated at 500° C. for 30 minutes to burn away the vacuum-resistantbag. Thus released from vacuum, the space between the two cylinders isfilled with the heat insulator, thereby producing a heat insulatedexhaust pipe.

In the conventional practice of wrapping the heat insulator around thesurface of the inner cylinder and simply forcing the inner cylinder intothe outer one, a heat insulator about 4 mm thick at the most isavailable for the manufacture of the above-mentioned heat insulatedexhaust pipe. Thus in comparison between the products of Examples 3, 4and the conventional product in a vibration test (the conditions beingthe same as above), the conventional product was found to be extremelyone-sided, resulting in a heavy drop in its heat insulating effect,whereas the products according to the present invention were free fromsuch a defect.

As described above, the present invention makes the filling of the heatinsulator easy so that the fibrous heat insulator can be filled to suchhigh density that the filled layer can exhibit excellent anti-vibrationcharacteristics and heat insulating properties. Moreover, the presentinvention eliminates the sanitary problem of fine particles of thefibrous heat insulator becoming scattered into the air at the work site,and many other benefits accrue from the present invention.

What is claimed is: .[.1. Method of manufacturing a heat insulator whichcomprises the steps of introducing between an outer casing and an innercasing an insert comprising a mass of fibrous material which has beencompressed by vacuum-packing it in a hermetically sealed bag, and thenunsealing said bag to permit said material to expand within saidcasing..].
 2. Method .[.as claimed in claim 1.]. .Iadd.of manufacturinga heat insulator .Iaddend.which comprises the .[.step.]. .Iadd.steps.Iaddend.of .[.first.]. wrapping .[.said.]. .Iadd.a hermetically.Iaddend.sealed bag .[.containing said mass of fibrous material.]. aboutan inner casing to form .[.the.]. .Iadd.an .Iaddend.insert .[.introducedinto said outer casing.]., .Iadd.said sealed bag containing a mass offibrous material which has been compressed by vacuum-packing saidfibrous material in the sealed bag; and then introducing said insertinto an outer casing; and then unsealing said bag to permit said fibrousmaterial to expand between the inner casing and the outer casing;wherein the fibrous material is caused to fill the space between innerand outer cylindrical casing encircling a passage for the gas exhaustedfrom an automotive engine.Iaddend.. .[.3. Method as claimed in claim 1which said bag is sealed in such a way that it may be unsealed by theapplication of an amount of heat insufficient to damage said fibrousmass and casing, and comprising the step of applying said amount of heatto said insulator after said bag has been introduced into saidcasing..]. .[.4. Method as claimed in claim 1 in which said bag is madeof a material which is destroyed by the application thereto of an amountof heat insufficient to damage said casing and fibrous mass, andcomprising the step of applying said amount of heat to said insulatorafter said bag has been introduced into said casing..]. .[.5. Method asclaimed in claim 1 in which said fibrous material is selected from thegroup consisting of rock wool, silica fiber, ceramic fiber, and mixturesthereof..].
 6. Method as claimed in claim 1 in which said bag is madefrom a plastic selected from the group consisting of nylon,polyethylene, polypropylene, and combinations thereof..]. .[.7. Methodas claimed in claim 1 in which the fibrous mass is caused to fill thespace between inner and outer cylindrical casing encircling a passagefor the gas exhausted from an automotive engine..]. .Iadd.8. Method asclaimed in claim 2 in which said bag is sealed in such a way that it maybe unsealed by the application of an amount of heat insufficient todamage said fibrous mass and casing, and comprising the step of applyingsaid amount of heat to said insulator after said bag has been introducedinto said casing. .Iaddend. .Iadd.9. Method as claimed in claim 2 inwhich said bag is made of a material which is destroyed by theapplication thereto of an amount of heat insufficient to damage saidcasing and fibrous mass, and comprising the step of applying said amountof heat to said insulator after said bag has been introduced into saidcasing. .Iaddend. .Iadd.10. Method as claimed in claim 2 in which saidfibrous material is selected from the group consisting of rock wool,silica fiber, ceramic fiber, and mixtures thereof. .Iaddend. .Iadd.11.Method as claimed in claim 2 in which said bag is made from a plasticselected from the group consisting of nylon, polyethylene,polypropylene, and combinations thereof. .Iaddend. .Iadd.12. Method ofmanufacturing a heat insulator which comprises the steps of wrapping ahermetically sealed bag about an inner casing to form an insert, saidsealed bag containing a mass of fibrous material which has beencompressed by vacuum-packing said fibrous material in the sealed bag;and then introducing said insert into an outer casing; and thenunsealing said bag to permit said fibrous material to expand between theinner casing and the outer casing. .Iaddend. .Iadd.13. Method ofmanufacturing a heat insulator comprising the steps of:I. wrapping ahermetically sealed bag about an inner cylindrical casing to form aninsert, said sealed bag containing a mass of fibrous material which hasbeen compressed by vacuum-packing it in said sealed bag, said sealed bagbeing made from a plastic selected from the group consisting of nylon,polyethylene, polypropylene and combinations thereof, and said fibrousmaterial being selected from the group consisting of rock wool, silicafiber, ceramic fiber and mixtures thereof; and then II. introducing saidinsert into an outer cylindrical casing; and then III. applying anamount of heat sufficient to destroy said sealed bag, the amount of heatbeing insufficient to damage said casing and fibrous material; wherein,said mass of fibrous material is caused to fill the space between innerand outer cylindrical casing encircling a passage for the gas exhaustedfrom an automotive engine. .Iaddend. .Iadd.14. Method of manufacturing aheat insulator comprising the steps of:I. wrapping a hermetically sealedbag about an inner cylindrical casing to form an insert, said sealed bagcontaining a mass of fibrous material which has been compressed byvacuum-packing it in said sealed bag, said sealed bag being made from aplastic selected from the group consisting of nylon, polyethylene,polypropylene and combinations thereof, and said fibrous material beingselected from the group consisting of rock wool, silica fiber, ceramicfiber and mixtures thereof; and then II. introducing said insert into anouter cylindrical casing; and then III. applying an amount of heatsufficient to unseal said sealed bag, the amount of heat beinginsufficient to damage said casing and fibrous material; wherein saidmass of fibrous material is caused to fill the space between inner andouter cylindrical casing encircling a passage for the gas exhausted froman automotive engine. .Iaddend.